الفهرس 
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Abstract
This study was carried out during the two consecutive experimental seasons 2008 and 2009 on forty- eight uniform in vigour 43- years old Washington Navel orange trees onto sour orange rootstock. The selected trees are grown in a private citrus orchard located at Citrus Research Farm, Sids Horticultural Research Station Beba district, Beni Suef Governorate (about 125 km southern Cairo). The trees planted according to the square arrangement method at a spacing of 6 × 6 meters. The soil of the orchard is well drained clay with a water table not less than two meters deep. Surface irrigation system was carried out using Nile water.
The scope of the present study was examining the effect of seaweed extract, boron and citric acid as possible means to improve fruit setting and the yield quantitively and qualitatively of Washington Navel orange trees.
This investigation was consisted from sixteen treatments from two factors (A & B). The first factor (A) comprised from four concentrations of seaweed extract namely a1) 0.0 %, a2) 0.05 %, a3) 0.1 % and a4) 0.2 %. The second factor (B) contained four boric acid and citric acid treatments i.e. b1) 0.0 %, b2) Spraying boric acid at 0.05 %, b3) Spraying citric acid at 0.2 % and b4) Spraying boric acid at 0.05 % + citric acid at 0.2 %. Therefore, this experiment included sixteen treatments (A = 4 × B = 4). Each treatment replicated three times, one tree per each (48 trees for achieving of this study). Algifert compound containing seaweed extract, boric acid (17 % B) and citric acid as antioxidant compound (100 %) were sprayed four times at growth start (last week of Feb.), just after fruit setting (1st week of April) and at one month intervals (1st week of May and June, respectively).
Statistical analysis was done using randomized complete block design in splite plot arrangement. Each treatment was replicated three times, one Washington Navel orange tree per each. Each block contained the investigated sixteen treatments. The four concentrations of seaweed extract and the four boric acid and citric acid treatments occupied the main and subplots, respectively.